The present invention relates to a family of novel metal alloys having a lighter weight and better oxidation resistance than alloys with comparable melting range, and having a tensile strength greater than wrought Ni-base alloys at high temperature. It also relates to structures in which such novel alloys are a part.
The invention additionally relates to body centered cubic metal structures in which the metal is ductile at room temperature, but which retain significant tensile strength at high temperatures.
It is known that niobium base alloys have useful strength in temperature ranges at which nickel and cobalt base superalloys begin to show incipient melting. This incipient melting temperature is in the approximately 2300.degree. to 2400.degree. F. range. The use of the higher melting niobium base metals in advanced jet engine turbine hot sections would allow higher metal temperatures than are currently allowed. Such use of the niobium base alloy materials could permit higher flame temperatures within the engines and would also permit production of greater power at greater efficiency. Such greater power production at greater efficiency would be at least in part due to a reduction in cooling air requirements. Most niobium base alloys, particularly those which are commercially available, are subject to oxidation to a degree which makes their use in high temperature air atmospheric environments unacceptable because of the lack of reliable coatings.
Certain alloys having a niobium-titanium base have much lower densities of the range 6-7.3 grams per cubic centimeter. A group of such alloys are the subject matter of commonly owned U.S. Pat. Nos. 4,956,144; 4,990,308; 5,006,307; 5,019,334; and 5,026,522. Such alloys can be formed into parts which have significantly lower weight than parts formed of niobium based alloys or than presently employed nickel and cobalt superalloys as these superalloys have higher densities ranging from about 8 to about 9.3 grams per cubic centimeter.
One additional patent, U.S. Pat. No. 4,931,254, concerns an alloy having the following composition in atom percent:
______________________________________ Concentration Ingredient Range ______________________________________ niobium balance titanium 40-48% aluminum 12-22% hafnium 0.5-6% chromium 3-8% ______________________________________
Commonly owned U.S. Pat. No. 4,904,546 concerns an alloy system in which a niobium base alloy is protected from environmental attack by a surface coating of an alloy highly resistant to oxidation and other atmospheric attack.
In devising alloy systems for use in aircraft engines the density of the alloys is, of course, a significant factor which often determines whether the alloy is the best available for use in the engine application. The nickel and cobalt based superalloys also have much greater tolerance to oxygen exposure than the commercially available niobium based alloys. The failure of a protective coating on a nickel or cobalt superalloy is a much less catastrophic event than the failure of a protective coating on many of the niobium based alloys and particularly the commercially available niobium based alloys. The oxidation resistance of the niobium based alloys of the above commonly owned patents is intermediate between the resistance of commercial Nb base alloys and that of the Ni- or Co-based superalloys.
While the niobium titanium based alloys of the above commonly owned patents are stronger than wrought nickel or cobalt based superalloys at high temperatures, they are much weaker than cast or directionally solidified nickel based superalloys at these higher temperatures. However, for many engine applications, structures formed by wrought sheet fabrication are used, since castings of sheet structures cannot be produced economically in sound form for these applications.
What is highly desirable in general for aircraft engine use is an alloy which can be formed into a structure which has a combination of lower density, higher strength at higher temperatures, good ductility at room temperature, and higher oxidation resistance.
A number of articles have been written about use of refractory metals in high temperature applications. These articles include the following:
(1) S. T. Wlodek, "The Properties of Cb-Ti-W Alloys, Part I", Oxidation Columbium Metallurgy, D. Douglass and F. W. Kunz, eds., AIME Metallurgical Society Conferences, vol. 10, Interscience Publishers, New York (1961) pp. 175-204.
(2) S. T. Wlodek, "The Properties of Cb-Al-V Alloys, Part I", Oxidation ibid., pp. 553-584 .
(3) S. Priceman and L. Sama, "Fused Slurry Silicide Coatings for the Elevated Temperature Oxidation of Columbium Alloys", Refractory Metals and Alloys IV - TMS Conference Proceedings, French Lick, IN, Oct. 3-5, 1965, vol . II, R. I . Jaffee, G. M. Ault, J. Maltz, and M. Semchyshen, eds., Gordon and Breach Science Publisher, New York (1966) pp. 959-982.
(4) M. R. Jackson and K. D. Jones, "Mechanical Behavior of Nb-Ti Base Alloys", Refractory Metals: Extraction, Processing and Applications, K. C. Liddell, D. R. Sadoway, and R. G. Bautista, eds., TMS, Warrendale, PA (1990) pp. 311-320.
(5) M. R. Jackson, K. D. Jones, S. C. Huang, and L. A. Peluso, "Response of Nb-Ti Alloys to High Temperature Air Exposure", ibid., pp. 335-346.
(6) M. G. Hebsur and R. H. Titran, "Tensile and Creep Rupture Behavior of P/M Processed Nb-Base Alloy, WC-3009", Refractory Metals: State-of-the-Art 1988, P. Kumar and R. L. Ammon, eds., TMS, Warrendale, PA (1989) pp. 39-48.
(7) M. R. Jackson, P. A. Siemers, S. F. Rutkowski, and G. Frind, "Refractory Metal Structures Produced by Low Pressure Plasma Deposition", ibid., pp. 107-118.